1. Field of the Invention
The invention relates to power supply and, in particular, to power supply for real-time clock generation.
2. Description of the Related Art
Most modern electronic systems are provided with real-time clocks that keep track of time even when an electronic system is turned off. Typically, real-time clocks run on a special battery not connected to a normal power supply.
FIGS. 1A and 1B are schematic diagrams of a conventional power supply for real-time clock generation disclosed in U.S. Pat. No. 6,016,019. In FIG. 1A, there are two power sources, a system power VSYS and a battery power VBATT, for real-time clock generation. A regulator 102 receives the battery power VBATT and generates a reference voltage VREF. A power selection circuit PS comprises an amplifier 26, an inverter 28, and transistors 30 and 32. When the system power VSYS exceeds the reference voltage VREF, the power selection circuit PS selects VSYS as a power supply VPP for real-time clock (RTC) circuits. When the system power VSYS is lower than the reference voltage VREF, the power selection circuit 102 selects VREF as the power supply VPP for real-time clock (RTC) circuits. As a result, power remains to keep time information of a system even when the system power VSYS is lost.
FIG. 2 is a schematic diagram of another conventional power supply for real-time clock generation disclosed in U.S. Pat. No. 5,905,365. Operating principles thereof are similar to U.S. Pat. No. 6,016,019 and only differ in that the power selection circuit in the disclosure of U.S. Pat. No. 5,905,365 is a diode. The voltage supplied to the RTC circuit is lower than a system power VCC or a battery power BATT by voltage drop of the diode. When the system power VCC is lower than the battery power BATT, the diode D1 is reverse-biased and the diode D2 forward-biased. Thus, the battery power BATT supplies power to the RTC circuit RTC when the system power can not supply enough power to the RTC circuit RTC.
In the conventional power supplies for real-time clock generation, voltage of the system power VSYS or VCC is typically higher or even the highest in the system. In advanced semiconductor process technology, RTC circuits, however, are typically implemented with core devices having lower voltage endurance. Therefore, there is a need to have a new power supply which can provide sufficient power to an RTC circuit without exceeding the low voltage endurance.